Field of the Invention
The invention concerns a method for movement correction of magnetic resonance measurement data, as well as a magnetic resonance apparatus and a storage medium encoded with program code for implementing such a method.
Description of the Prior Art
In a magnetic resonance apparatus, also called a magnetic resonance tomography system, a subject to be examined, in particular a patient, is exposed with the use of a basic field magnet to a high basic magnetic field, for example 1.5 or 3 or 7 tesla. In addition, gradient pulses are emitted by a gradient coil arrangement. Radio-frequency pulses, for example excitation pulses, are then emitted via a radio-frequency antenna unit by suitable antenna arrangements, which cause nuclear spins of specific atoms that are excited resonantly by these radio-frequency pulses to be flipped by a defined flip angle in relation to the magnetic field lines of the basic magnetic field lines of the basic magnetic field. During relaxation of the nuclear spins, radio-frequency signals, called magnetic resonance signals, are emitted and then received by suitable radio-frequency antennas, and then further processed. The desired image data can be reconstructed from the raw data acquired in this manner.
For a specific measurement, a specific magnetic resonance sequence, also called a pulse sequence, is transmitted, which is composed of a sequence of radio-frequency pulses, for example excitation pulses and refocusing pulses, and gradient pulses that are suitably coordinated thereto, in different gradient axes along different spatial directions. Chronologically coordinated therewith, read-out windows are set that pre-specify the periods of time in which the induced magnetic induced magnetic resonance signals are acquired.
During magnetic resonance imaging, movement of the object under examination may occur. For example, a respiratory movement and/or cardiac movement of the object under examination may occur. It is also possible for arbitrary movements of limbs of the object under examination to occur. This movement of the object under examination can result in an unwanted change to the magnetic resonance measurement data acquired during the magnetic resonance imaging. For example, the occurrence of movement artifacts in the acquired magnetic resonance measurement data is conceivable. The movement can also reduce the quality of the acquired magnetic resonance measurement data.
It is particularly during magnetic resonance imaging that the movement of the object under examination can have a considerable influence on the measured magnetic resonance measurement data. This is because, in dependence upon the magnetic resonance sequence used, the recording of the magnetic resonance measurement data can require a longer time period of up to several minutes. It is also possible for a time series to be recorded during the magnetic resonance imaging. This means that the same two-dimensional or three-dimensional examination region is recorded multiple times at different time points one after the other. This can result in the depiction of a temporal change to a section of the anatomy of the object under examination in the magnetic resonance measurement data acquired in this way. A possible movement of the object under examination during the acquisition of the time series can complicate or impair the evaluation of the time series since, when the image position is kept constant during the acquisition of the time series, the movement causes the depicted anatomy of the object under examination to change over the time series.
Different methods are known for at least partial compensation of the movement of the object under examination during the acquisition of the magnetic resonance image data. One known method for at least partial compensation of the movement of the object under examination is prospective movement correction. In this context, prospective movement correction generally involves the detection and correction of a movement of the object under examination during the acquisition of the magnetic resonance measurement data. This enables the acquisition of the magnetic resonance measurement data to be adapted to the movement of the object under examination during the actual magnetic resonance imaging. For example, image-based navigators can be used to detect the movement of the object under examination during the acquisition of the magnetic resonance measurement data.
A further known method is retrospective movement correction, which typically compensates the movement of the object under examination in the magnetic resonance measurement data following the acquisition of the magnetic resonance measurement data. In this context, the magnetic resonance measurement data itself can be used for the movement correction of the magnetic resonance measurement data.